This invention relates to a method for making a dielectric ceramic body and more particularly wherein two powders of differently-doped barium titanates are mixed, formed into a body and sintered to maturity with only a partial reaction between the two types of barium titanate grains.
Pure polycrystalline barium titanate has a dielectric constant that is relatively unchanging with changes in temperature except for a large spike at 125.degree. C. This is the Curie temperature at which the dielectric constant may be as much as an order of magnitude larger than at room temperature. It is well known to dope barium titanate with Curie point shifters to move the spike and Curie temperature to a desired temperature at which a high dielectric constant is needed, e.g. 25.degree. C. Such downward Curie point shifters are niobium, lanthanum, strontium, zirconium. etc. On the other hand, a small doping addition of lead or of yttrium in the barium titanate will raise the Curie temperature.
Such a doped polycrystalline barium titanate body for use as a capacitor dielectric may be made by mixing the powders of pure barium titanate (or alternatively precursors thereof such as BaCO.sub.3 and TiO.sub.2) and a powder of a precursor, e.g. Nb.sub.2 O.sub.5, of the dopant cation, e.g. Nb.sup.+5, then compressing the mixture in a mold or otherwise forming a body of the powder mixture, and firing the body to cause the dopant to react with and enter substitutionally into the crystalline grains of the barium titanate. The "small" cation Nb.sup.+5 as the dopant will displace small titanium cations (Ti.sup.+4) in the crystal lattice. The "large" cation La.sup.+3 as the dopant will displace large barium cations (Ba.sup.+2) in the crystal lattice. Such dopant substitutions and their effects are more fully explained, for example, in the patent to Burn U.S. Pat. No. 4,283,753 issued Aug. 11, 1981 and assigned to the same assignee as is the present invention.
When the firing is long enough and hot enough to effect a thorough and homogeneous penetration of the dopant within each and all of the barium titanate grains, a high dielectric constant (K) may be achieved, but the temperature coefficient (TC) of the dielectric constant will vary greatly over the temperature ranges of commercial interest.
A much flatter TC is often needed, as is reflected in the widely employed commercial standard that is designated X7R calling for no greater change in the dielectric constant from the value at 25.degree. C. than .+-.15% over the temperature range of -55.degree. C. to 125.degree. C. Ceramic compositions of mainly barium titanate have been made that meet that standard.
An early one of such compositions is described in the patent to Prokopowicz U.S. Pat No. 3,231,799 issued Jan. 25, 1966 wherein a dopant, e.g. niobium, was reacted with barium titanate at a sintering for 1 hour at 2320.degree. F. (1271.degree. C.) using no glass containing sintering aid. It is recognized that the reaction (without flux) at sintering in the Prokopowicz' patent was incomplete, not having heated long enough, or alternatively hot enough, to permit a homogeneous dispersion of the niobium dopant throughout each grain, and furthermore there was essentially no grain growth.
It is postulated that each of the small Prokopowicz' grains has a high concentration of the niobium dopant near the grain surface, which concentration diminishes rapidly toward the grain center at which it is essentially zero. In this model the Curie point of the grain surface is lowest and the Curie temperature rises as the grain center is approached. Thus each and every barium titanate grain is doped in the same way and each has the same mildly varying dielectric constant as a function of its temperature.
This composition has the drawback, however, that the particular TC achieved is a strong function of the sintering conditions and of the start barium titanate grain size.
Another but recently disclosed composition is quite similar to that of Prokopowicz except for including a sintering aid to provide a lower sintering temperature. The addition of a flux or sintering aid is known to promote liquid phase sintering, whereby the barium titanate dissolves in the liquid flux and simultaneously recrystallizes. Such an aid, or flux, permits sintering at a lower temperature or otherwise speeds up the sintering process. Apparently here again, each and every barium titanate grain has the same mildly temperature varying dielectric constant as does every other grain
Another approach to making an X7R ceramic is to provide a ceramic body that is a mixture of two different kinds of ceramic grains and presumably having dissimilar Curie temperatures. A known example is comprised of a mixture of a rhombohedral phase crystalline material with a tetragonal phase crystalline material precipitated from a liquid flux at sintering from a mixture of start materials including compounds of barium, strontium, lead, niobium and a flux.
It is therefore an object of this invention to provide a partially reacted ceramic dielectric material exhibiting a high dielectric constant and a smooth temperature coefficient that are more predictable and more controllable in manufacture.